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SX-950 -- Noisy phono stage (AWF-011) after re-cap

Adorno

Adorno

New Member
Hi all -- first, a thanks to everyone posting information about these amps. My father had one while I was growing up and was very attached to it and the sound. It's nice to know I'm not alone in loving these old receivers and that there is perhaps more to them than just nostalgia.

I've just completed a re-cap/refurb of (my dad's old) amp -- mostly successfully -- with info from the forums here. One issue remains: There is a good bit of low level static (white/pink noise-like) on the phono channels. I'm out of ideas on my own and so thought I should try a post. Forgive me if this is basic stuff.

I replaced all transistors on the board, Q1,Q2 with KSA992 and Q3-6 with KSC1845. I *did not* use gain matched 992s for Q1 and Q2 on this board. I'm wondering if this might be the reason for the noise? (I don't have a lot of experience in this arena..)

Happy to post voltages and a parts list if it's helpful -- otherwise any and all debugging ideas beyond poking components with something non-conductive are welcome.
 
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OK - sounds like you have taken a few good steps here.

Welcome to AK.

Is the nature of the crackling completely identical in both channels of the phono section? If so, I would be looking at transistors in the powersupply that is feeding the phono section
 
Thank you! It is indeed identical in both channels.
I have the parts for Q5-10 on the power supply board but have not swapped them out yet - I'll give that a go and see if it improves. Again, perhaps a stupid question but are the off-board transistors (2SB557/2SD427) likely to affect this?
 
.. bit of an update. I spent some time going backwards through the work and found some pin voltages that weren't what they should be. Sure enough, I'd installed a transistor backwards. It seems to have improved the situation considerably, but there is still a bit of hiss if I turn the volume up, without a turntable connected.

I'm not sure if it's related so I will mention it here in case someone else happens to read this -- if I've got something connected and playing into one of the other inputs, it seems to pick up and amplify _that_ signal.. crosstalk, maybe? I went over my work on the switch board too, but all seems to be OK at this point.
 
Adorno said:
.. bit of an update. I spent some time going backwards through the work and found some pin voltages that weren't what they should be. Sure enough, I'd installed a transistor backwards. It seems to have improved the situation considerably, but there is still a bit of hiss if I turn the volume up, without a turntable connected.

I'm not sure if it's related so I will mention it here in case someone else happens to read this -- if I've got something connected and playing into one of the other inputs, it seems to pick up and amplify _that_ signal.. crosstalk, maybe? I went over my work on the switch board too, but all seems to be OK at this point.
Click to expand...

I'm not sure how loud you're turning it up to hear hiss, but hearing hiss on a phono input with nothing connected is normal unless the hiss is present at a low volume. By the time you turn the volume 1/2 up, some hiss will be present.

Now aux on the other hand, should be almost "dead" quiet.

Crosstalk can be normal as well.
 
Thanks Bruno - it's probably a normal amount of noise if that's the case. It's noticeable over about 1/2, and i'm picking up the crosstalk when the volume is up beyond 3/4. Aux is dead quiet as you mention. If that's the case, the board is probably functioning properly.

Anyone up for some help in troubleshooting a flaky left channel? :)
(kidding, mostly. I have some more checking to do before I embarass myself further.)
 
I just wanted to loop back and make some final comments in case someone is searching for simlar problems in the future. The actions I took were the following:

- Checked into the transistors supplying power to the phono board. I discovered one had been installed incorrectly. It's worth noting that you should double check the pinouts on transistors for these amps if you're replacing any as the original devices differ from modern suggested replacements..
- Remaining residual hiss seemed to disappear once a turntable was connected (thanks EastPoint/Bruno)
- Unrelated, I was getting some degradation of the left channel after about an hour of playing. I went through the power amp board and found .. not exactly a cold weld but a questionable one (somewhat degraded traces). After fixing that, the unit is now playing like a champ.

If you've landed here via searching and trying to solve a similar problem, this is a useful video for debugging an intermittent channel on an sx-950:

Thanks again to everyone who continues to post information on working on these receivers, nevermind answering my questions here!
 
Does to hurt to measure the bias voltages, compare against the schematic, to figure out if you installed the parts correctly.
 
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